The present invention relates to a method for protecting a matrix converter having nine bidirectional power switches, which are arranged in a 3xc3x973 switch matrix, against overvoltages, and to an active overvoltage protection device.
A matrix converter is a self-commutating direct converter. This self-commutating direct converter is a converter without an intermediate circuit. The arrangement of the electronic power switches in a 3xc3x973 switch matrix results in the three input phases being connected to the three output phases. This self-commutating direct converter has the advantage that its topology allows a feedback capability, and appropriately applied control results in sinusoidal power supply system currents. A semiconductor switch integrated in a diode bridge, on the one hand, and two back-to-back series-connected semiconductor switches on the other hand may be used as the bidirectional switches in the switch matrix. The two back-to-back series-connected semiconductor switches in a bidirectional power switch in the switch matrix are configured using either the common emitter mode or common collector mode topology. The embodiment of the bidirectional power switch with a semiconductor switch being embedded in a diode bridge is referred to as an embedded switch.
FIG. 1 shows a circuit diagram of a conventional bidirectional switch 2 in the common collector mode topology. For comparison, FIG. 2 shows a conventional bidirectional power switch 2 in the common emitter mode topology. These two bidirectional power switches 2 each have two semiconductor switches 4 and 6, which are connected back to back in series. In FIG. 1, these two semiconductor switches 4 and 6 are connected back-to-back in series such that the two collector terminals are electrically conductively connected to one another. This back-to-back series circuit formed by the two semiconductor switches 4 and 6 is therefore also referred to as the common collector mode. In FIG. 2, the two semiconductor switches 4 and 6 are connected back-to-back in series such that their emitter terminals are electrically conductively connected. Since the emitter terminals are linked, this circuit is referred to as the common emitter mode. Semiconductor switches which can be turned off, in particular Insulated Gate Bipolar Transistors (IGBT), are used as the semiconductor switches 4 and 6, and each have a reverse diode. The internal topology can be seen from the accessible terminals of the bidirectional power switch 2. In the bidirectional power switch 2 in the common collector mode topology as shown in FIG. 1, the terminals E1, E2, G1 and G2 are accessible on the power switch 2. In contrast to this, in the bidirectional power switch 2 in the common emitter mode topology as shown in FIG. 2, the terminals C1, C2, G1 and G2 are accessible. In addition, these bidirectional power switches 2 have auxiliary terminals EH1 and EH2, which each form a control terminal.
FIG. 3 shows in more detail a circuit diagram of a conventional bidirectional power switch 2 in the embedded switch topology. This bidirectional power switch 2 has a semiconductor switch 5 which can be turned off, in particular an Insulated Gate Bipolar Transistor (IGBT), which is arranged in a diode bridge. The collector side of this semiconductor switch 5 is electrically conductively connected to cathode terminals of two diodes, and its emitter side is electrically conductively connected to anode terminals of two further diodes in the diode bridge. The free terminals of these diodes each form an input and output terminal for the bidirectional power switch 2.
Driving of the semiconductor switches 4 and 6 and of the semiconductor switch 5 in the bidirectional power switch 2 in the matrix converter in each case switches on one current path in a specific direction. If both the semiconductor switches 4 and 6 are actuated, then this allows current to flow in both directions, so that a reliable electrical connection is produced between one input phase and one output phase. If only one semiconductor switch 4 or 6 in the bidirectional power switch 2 in the matrix converter is actuated when the bidirectional power switch 2 is in the common collector mode or common emitter mode topology, respectively, then this results in a connection for only one preferred current direction. One phase of the matrix converter is an arrangement of three bidirectional power switches, which produces a connection from the three power supply system phases to in each case one of the output phases.
Since the matrix converter has no passive freewheeling circuits, in the same way as a voltage intermediate circuit converter, then, particularly in the case of a pulse inhibitor generated on the basis of an EMERGENCY OFF (with the actuate pulses to all the semiconductor switches being switched off), a high reverse voltage occurs across the semiconductor switches owing to the inductances in the circuit. These overvoltages can also occur as a result of a failure of the actuate for the bidirectional power switches. The output current is interrupted in each of these situations that have been mentioned. The interruption in the output circuit in conjunction with the inductances in the circuit causes the overvoltages, which can lead to destruction of the semiconductor switches.
A general overvoltage protection device is known from the publication xe2x80x9cNovel Solutions for Protection of Matrix Converter to Three Phase Induction Machinexe2x80x9d, printed in the Proceedings of the xe2x80x9cIEEE Industry Applications Societyxe2x80x9d New Orleans, La., October 5-9, 1997, pages 1447 to 1454. This overvoltage protection device has two 6-pulse diode bridges, which are linked to one another on the DC voltage side by means of a capacitor. On the AC voltage side, one of the 6-pulse diode bridges is connected to the input terminals of the matrix converter. The other diode bridge is connected on the AC voltage side to the output terminals of the matrix converter. A resistor is connected electrically in parallel with the capacitor, and discharges the capacitor. The input terminals of the matrix converter are also connected to an LC filter, whose input side is connected to a three-phase power supply system. This LC filter, which is also referred to as an input filter, keeps pulse-frequency harmonics away from the power supply system. The size of this filter depends on the pulse repetition frequency of the matrix converter.
Any overvoltages are rectified by the diode bridges and passed to the capacitor. A precharging circuit is required for the capacitor for this overvoltage protection device, which is also disclosed in U.S. Pat. No. 4,697,230. This precharging circuit is required in order that no inrush current surges or overvoltages occur at twice the power supply system voltage when the matrix converter is switched on. Overvoltages such as these cause high surge currents, which have to be carried by the diodes in the diode bridge. The resistor is designed such that its resistance ensures that a predetermined amount of energy is discharged from the capacitor.
An overvoltage protection device with two 6-pulse diode bridges is also known from the publication xe2x80x9cPerformance of a two Steps Commutated Matrix Converter for AC-Variable-Speed Drivesxe2x80x9d printed in the Proceedings of EPE""99, Lausanne, September 1999, pages 1 to 9. Each of these two diode bridges has a capacitor on the DC voltage side. These two capacitors are electrically connected in parallel. A Zener diode and a pulse resistor are electrically connected in parallel with these two capacitors, thus limiting the voltage on the capacitors to a predetermined value. Furthermore, each bidirectional power switch has a varistor and two back-to-back series-connected Zener diodes, which limit the overvoltages across the bidirectional power switch.
A number of varistors are provided as an overvoltage protection device in the publication xe2x80x9cA Matrix Converter without Reactive Clamp Elements for an Induction Motor Drive Systemxe2x80x9d, by Axel Schuster, printed in IEEE, 1998, pages 714 to 720. A varistor is electrically connected in parallel with each semiconductor switch in each bidirectional power switch in the 3xc3x973 switch matrix. These varistors protect the 18 semiconductor switches in the nine bidirectional power switches against overvoltages.
When this overvoltage protection device is used, the junction point of the two collector terminals of the two back-to-back series-connected semiconductor switches must be passed to the exterior when the bidirectional power switch is in the common collector mode. It is also possible for the bidirectional power switch to be formed from individual semiconductor components. A varistor can be electrically connected in parallel with each semiconductor switch in a bidirectional power switch only if the collector terminals, or their junction point, are/is accessible.
A voltage clamping circuit is known from the publication xe2x80x9cBeschaltung von SIPMOS-Transistorenxe2x80x9d [Circuitry for SIPMOS transistors], printed in xe2x80x9cSiemens Componentsxe2x80x9d, Volume 22, Issue 4, 1984, pages 157 to 159. This voltage clamping circuit 8 is illustrated as semiconductor switch 4 in FIG. 4. This voltage clamping circuit 8 includes a Zener diode 10, in particular a high-voltage Zener diode which is also referred to as a Transil diode, and a decoupling diode 12. This voltage clamping circuit 8 is connected between the collector terminal C and the gate terminal G of the semiconductor switch 4. An Insulated Gate Bipolar Transistor (IGBT) with a reverse diode is provided as the semiconductor switch 4. The decoupling diode 12 isolates the voltage clamping circuit 8 from the gate terminal G of the semiconductor switch 4 when the semiconductor switch 4 is switched on. When the semiconductor switch 4 is switched off, as soon as its collector/emitter voltage exceeds the sum of the Zener voltage of the Transil diode, the threshold voltage of the decoupling diode 12 and the gate/emitter threshold voltage, the semiconductor switch 4 is actuated automatically. Any overvoltage which occurs across the semiconductor switch 4 is thus actively limited by it, although losses occur in the semiconductor switch 4 and in the Transil diode 10.
This active overvoltage protection device may be used directly in a bidirectional power switch in the common emitter mode topology (FIG. 2). This means that each of the two semiconductor switches 4 and 6 in the directional power switch 4 in the common emitter mode has a voltage clamping circuit 8 connected electrically in parallel with its collector/gate junction. This can also be done without any major complexity, since the required terminalsxe2x80x94collector terminal C and gate terminal Gxe2x80x94are accessible.
This known voltage clamping circuit 8 for a bidirectional power switch in the common collector mode cannot be used without modifications. Hence, the common collector connection must be passed to the exterior from the bidirectional power switch 2.
It would therefore be desirable to obviate prior art shortcomings and to protect a matrix converter against overvoltages cost-effectively and with little complexity.
According to one aspect of the present invention, a method includes determining the presence of an overvoltage, automatically actuating all bidirectional power switches which are at risk in the matrix converter. This automatic actuation of the bidirectional power switches actively limits the overvoltage to a predetermined value so that the overvoltage cannot destroy any bidirectional power switches in the matrix converter. With this method according to the invention, the connection of the matrix converter on which an overvoltage occurs is irrelevant, with the only factor of interest being that an overvoltage has occurred. As soon as this is detected, all the bidirectional power switches are actuated in such a way that a detected overvoltage is actively limited.
In one advantageous method, the potentials of the control terminals of all the bidirectional power switches in the matrix converter are evaluated rather than the potentials at the input and/or output terminals of the matrix converter. The advantageous method thus differs only by the location at which an overvoltage is detected, but not in the active limiting of an overvoltage which has been determined.
An active overvoltage protection device according to the invention has at least one rectifier circuit, at least one high-voltage Zener diode and at least one diode circuit, which has a number of diodes with a high blocking capability, with the cathode side of each high-voltage Zener diode being connected to a corresponding output of a rectifier circuit and its anode side being connected to a corresponding input of the diode network. The inputs of the rectifier circuits are connected to input and/or output terminals of the matrix converter or to the input terminals and at least one output terminal of the matrix converter. The cathode sides of the diodes with a high blocking capability in the diode circuit are each connected to one control terminal of the bidirectional power switches in the matrix converter.
In one advantageous embodiment of the active overvoltage protection device, this device is converter-oriented. This means that all the input and output terminals of the matrix converter are connected to the inputs of a rectifier circuit. Furthermore, only one high-voltage Zener diode is used, to whose anode the input of the diode circuit is connected. This converter-oriented refinement of the active overvoltage protection device means that only one high-voltage Zener diode is required.
In a further advantageous embodiment of this advantageous embodiment of the active overvoltage protection device, three diodes with a high blocking capability in the diode circuit are replaced by a diode network comprising three Zener diodes with a low blocking capability and one diode with a high blocking capability. The anode sides of the three low blocking capability Zener diodes are connected to the cathode of the high blocking capability diode, whose anode side is connected to the high-voltage Zener diode. This refinement further reduces the required number of high blocking capability diodes.
The embodiment of the active overvoltage protection device may also be designed on a phase-oriented basis, which then means that three high-voltage Zener diodes and three rectifier circuits are required. In this phase-oriented embodiment of the active overvoltage protection device, three high blocking capability diodes in the diode circuit can each once again be replaced by a diode network comprising three low blocking capability Zener diodes and one high blocking capability diode.
Furthermore, the converter-oriented or phase-oriented embodiment of the active overvoltage protection device may be subdivided depending on the direction of the current flow in the matrix converter. This means that the active overvoltage protection device may be designed jointly or separately on a phase-oriented or converter-oriented basis, and for the forward and return directions. Most of the high-voltage Zener diodes, namely six of them, are used for a phase-oriented embodiment with the forward and return directions separated.
The choice of the embodiment of the active overvoltage protection devices depends on the design of the matrix converter. The bidirectional power switches may all be accommodated in one module. These may also be integrated phase-by-phase in a respective module, or may also be integrated in one module in each case on the power supply system side and load side. Since the design of the active overvoltage protection device is dependent on the design of the matrix converter, parts of the overvoltage protection device may be located in the immediate vicinity of the modules of the converter. This results in a low-inductance link between the active overvoltage protection device and the matrix converter.